1. Field of the Invention
The present invention relates to a method for forming a film.
2. Description of the Prior Art
Films have been heretofore deposited by various processes such as CVD (chemical vapor deposition), sputtering, MBE (molecular beam epitaxy), and the like. In plasma enhanced CVD (referred to simply hereinafter as plasma CVD), the use of high frequency excitation, microwave excitation, hybrid resonance and the like has been developed. Particularly in the plasma CVD process which utilizes a resonance with a magnetic field (referred to as xe2x80x9cplasma CVD in magnetic fieldxe2x80x9d, hereinafter), the development thereof has actively taken place and, because of its high film deposition efficiency which results from the use of a high density plasma, its diversification in application has been expected. In the practical film deposition in the presence of a magnetic field, however, a difficulty has been encountered to deposit uniform films on an irregular surface without being influenced by such surface irregularity. This difficulty has hindered practical progress of the microwave CVD in magnetic field in the industries. The fact that a plasma CVD in magnetic field consumes an enormous amount of energy at its operation also is a bar to its gaining popularity in the industrial field. A diamond-like carbon (DLC) film can be uniformly deposited on a substrate as large as 10 cm or more in diameter by the use of microwave plasma CVD in magnetic field. In the deposition of such DLC films by this process, the diamond nuclei formed in the vapor phase are trapped on the substrate upon their contact with the substrate. Thus, the DLC film grows spread in a tapered form from each nucleus, and results in a film having poor adhesion with the substrate. Furthermore, since the diamond crystals grow in a tapered form from a diamond nucleus center trapped on the substrate, a compression stress accumulates around the grain boundaries within the DLC film. The poor adhesion of the film with the substrate and the compression stress within the film have constituted a hindrance to the practical use of DLC films.
An object of the present invention is to provide a process of depositing uniform films.
Another object of the present invention is to provide a process of depositing films with small power consumption.
Still another object of the present invention is to provide a process of depositing films which have excellent adhesion with substrates.
The foregoing objects and other objects have been achieved by depositing films by a plasma CVD process which takes advantage of the interaction between a magnetic field and an electric field, e.g. a high frequency electric field, induced by supplying an electric energy intermittently, or of that between a magnetic field and an electric field, e.g. a high frequency electric field, induced by supplying thereto an electric energy intermittently and a stationary electromagnetic energy continuously which are superposed upon each other. The magnetic field may be generated by supplying an electric energy intermittently. Alternatively, the magnetic field may be obtained by supplying either a DC current or a pulsed current to a Helmholtz coil. Furthermore, rise and decay of the pulsed current for generating the magnetic field intermittently and those of the electric power for generating the electric field intermittently may be synchronized with each other. In a typical embodiment, a microwave electric energy is supplied to generate the high frequency electric field.
In FIGS. 3(A), 3(B), and 3(C) are given examples of time versus power (effective value of power). FIG. 3(A) shows a shape having two different peak values. Such a power is particularly effective in increasing production of substances over a certain threshold value while suppressing the production of substances having an energy of production lower than the threshold value. FIG. 3(B) shows time versus power (effective value of power) of a wave obtained by superposing a high frequency electric wave supplied intermittently upon a low power electromagnetic stationary wave supplied continuously, wherein the initial waves have the same frequency. FIG. 3(C) also shows time versus power (effective value of power)of a wave obtained by superposing a high frequency electric wave supplied intermittently upon a low power electromagnetic stationary wave supplied continuously, however, the frequency of the initial waves are differed. The plasma CVD of the present invention is referred also to as a pulsed plasma CVD hereinafter since the power has a pulse shape as shown in FIGS. 3(A) to 3(C). The use of waves obtained by the superposition enables rapid deposition of the films, and is useful when a stable plasma cannot be obtained only by an intermittently supplied wave due to the structural allowance of the apparatus or to the conditions restricting the film deposition process. Thus, from the characteristic of a pulsed plasma CVD which enables a uniform formation of nuclei for film growth on the surface of substrates, the process enables deposition of a highly homogeneous film on an article having an irregular surface on one hand; on the other hand, from the fact that a high electric power can be concentrated at a pulse peak as compared with a stationary continuous power, the film deposition can be carried out at an increased efficiency.
To obtain a film of uniform thickness extended over a large area on a substrate, the film deposition is conducted in an apparatus the inner pressure of which is elevated to a range of from 0.03 to 30 Torr, preferably, from 0.3 to 3 Torr, using a high density plasma taking advantage of hybridized resonance. Since the pressure is maintained high, the mean free path of the reactive gas is shortened to a range of from 0.05 mm to several millimeters, particularly to 1 mm or less. This facilitates dispersion of the reactive gas to various directions, which is advantageous for depositing films on the sides of the articles having irregular surfaces. Thus, the rate of film growth is accelerated.
The article to be coated with a film is placed either in a hybridized resonance environment or in an activated environment remote from the hybridized resonance environment, to thereby coat the surface thereof with the reaction product. To achieve efficient coating, the article is located in the region at which a maximum electric field intensity of the microwave power can be obtained, or in the vicinity thereof. Furthermore, to generate and maintain a high density plasma at a pressure as high as in the range of from 0.03 to 30 Torr, an ECR (electron cyclotron resonance) should be generated in a columnar space under a low vacuum of 1xc3x9710xe2x88x924 to 1xc3x9710xe2x88x925 Torr and a gas, a liquid, or a solid is then introduced into the columnar space to produce a plasma, which is then maintained under a high pressure in the range of from 0.03 to 30 Torr, preferably from 0.3 to 30 Torr, so as to obtain a space having a highly concentrated product gas, said concentration per volume being about 102 to 104 times as large as the gas concentration normally used in a conventional ECR CVD process. By thus realizing the particular environment, the film deposition of a material which undergoes decomposition or reaction only at such a high pressure becomes possible. The particular films which can be obtained include carbon films, diamond films, i-carbon (carbon films containing diamonds or microcrystal grains), DLC (diamond-like carbon films), and insulating ceramics, and metallic films, in particular films of metal having high melting point.
In summary, the process according to the present invention utilizes plasma glow discharge and comprises a known microwave plasma CVS process to which a magnetic field is added to utilize the interaction of the magnetic field with the high frequency (micro wave) electric field. However, the ECR conditions are omitted from the process. The process according to the present invention conducts the film deposition in a hybridized resonance space using a high density plasma having a high energy, under a high pressure in the range of from 0.03 to 30 Torr. In the process according to the present invention, the plasma excitation is carried out with a pulsed wave or a combination of a pulsed wave and a stationary continuous wave, as set forth above, under a high energy state in the hybridized resonance space to thereby generate active species at an increased amount and also to effect homogeneous nuclei formation on the surface of the substrate. This enables the formation of a thin film material at an excellent reproducibility.
The power is supplied in pulses, as mentioned earlier, at an average power of from 1.5 to 30 KW with a peak pulse about three times the average power. The primary pulse should be supplied at a period of from 1 to 30 ms, preferably from 5 to 8 ms. Since the intensity of the magnetic field can be varied as desired, it is another characteristic of the process according to the present invention that the resonance condition can be set for not only the electrons but also for a specified ion.
In the deposition of a DLC film, for example, a pulsed wave having relationship between time and power (effective value of power) as shown in FIG. 4 can be applied. Preferably, the bonding within the DLC film is in sp3 hybridization. The ratio of the dissociation energy for Sp3 hybridization to that for sp2 hybridization is 6:5. In FIG. 4, it can be seen that the first peak 30 is 6/5 times as high as the second peak 31. In this case, the energy for the first peak 30 is preferably smaller than the dissociation energy of sp3 hybridization but maintained higher than the dissociation energy of sp2 hybridization, so as not to break the Sp3 hybridization bonding but to promote breakage of sp2 hybridization bonding. More specifically, for example, the energy of the first peak is set in the range of from 5 to 50 KW, and that of the second peak is set in the range of from 4.1 to 46 KW. Furthermore, in a pulsed high frequency plasma CVD, the nucleus formation is activated while the growth of the formed nuclei is suppressed. Such a phenomena results in a uniform formation of crystal nuclei over the substrate, which is followed by a growth into a DLC film composed of columnar crystals 29, said crystals being substantially one direction oriented toward the upper direction, such as shown in FIG. 5. Thus, a DLC film having a uniform crystal structure and dominant in sp3 hybridization can be deposited at a high reproducibility, free from problems frequently encountered in conventional processes, such as the stress due to tapered film growth and the peeling off of the deposited film induced therefrom. The pulsed wave power may be acicular pulse power, as well as the powers shown in FIGS. 6(A) and 6(B).
In another embodiment according to the present invention, a light (such as an ultraviolet (UV) light) may be simultaneously irradiated to the activated species to maintain the activated state for a longer duration. That is, the process comprises irradiating a light (e.g., a UV light) simultaneously with the generation of a high density plasma by the interaction of the pulsed microwave and the magnetic field, so that atoms excited to a high energy state can survive even at locations 10 to 50 cm distant from the area at which the maximum electric field intensity of the microwave power is obtained, i.e., the area at which a high density plasma is generated, since the high energy state is sufficiently maintained even on the surface of the article. This process enables deposition of a thin film over a further larger area. In the embodiment according to the present invention, a cylindrical column was established in such a space, and the article on which the film is to be deposited was provided inside the column to effect film deposition.
The generation of the microwave (at an average power in the range of from 1.5 to 30 KW) may be synchronized with the generation of the magnetic field using an electric power the pulse form of which is shown in FIG. 6(A). Alternatively, a multistep rectangular pulsed electric power as shown in FIG. 6(B) or that as shown in FIG. 6(C) may be used in place of the pulsed electric power illustrated in FIG. 6(A). A multistep rectangular pulsed wave may be applied, for example, in the deposition of a DLC film. Since the microwave and the magnetic field in this instance can be supplied with a peak power of about 5.0 to 50 KW if such a pulsed wave is used, the result is about 30 to 40% increased efficiency as compared with the case a plasma CVD apparatus is operated in a magnetic field with an input of an ordinary continuous wave at a power of from 1.5 to 30 KW. This enables reduction of power consumption of the plasma CVD apparatus operated in the presence of a magnetic field. The pulse duration of the pulsed wave should be in the range of from 1 to 10 ms, more preferably, from 3 to 6 ms.
It is also clarified that a film composed of more densified crystal grains can be uniformly deposited on the article irrespective of the surface irregularities of the article by applying a pulsed wave to a plasma CVD process in a magnetic field. This is also an advantage of the process according to the present invention.